1. Field of the Invention
The present invention relates to a heater comprising a honeycomb structure, an adsorbent (composed mainly of zeolite) or an adsorbent-catalyst composition coated on the honeycomb structure, the electrodes being fixed to the honeycomb structure. The present invention also relates to a catalytic converter comprising at least one main monolith catalyst and a heater arranged in a particular order; and a catalytic converter comprising a honeycomb heater, at least one main monolith catalyst and a zeolite adsorbent arranged in a particular order.
The above heater can be employed as a heater for domestic use, such as a hot air heater or as an industrial heater, such as a preheater used for control of automobile exhaust gas. The above catalytic converters can be employed for purification of automobile exhaust gas.
2. Description of the Prior Art
Catalytic converters used for purification of automobile exhaust gas or the like must be heated to a certain temperature or higher in order to exhibit their catalytic activities. Accordingly, when they are not sufficiently heated as in the start-up of an automobile, it is necessary to heat them.
A technique for heating a catalytic converter is proposed in Japanese Utility Model Application Laid-Open No. 67609/1988. This document discloses a catalytic converter comprising (a) a ceramic monolith catalyst and (b) an electrically heatable metal monolith catalyst provided upstream of the ceramic monolith catalyst (a) and in close vicinity thereto, consisting of a metal carrier and alumina coated thereon.
Of the harmful compounds (HC's, CO and NOx) present in automobile exhaust gases, particularly HC's (hydrocarbons) produce photochemical smog (oxidant). Accordingly, regulation therefor has been tightened, and proposals have been made to purify the HC's discharged in a large amount at the engine start-up, by utilizing zeolite for its adsorbability. For example, there has been proposed apparatuses for purification of automobile exhaust gas, arranged in the exhaust gas system of an automobile, which comprise (a) a purification catalyst and (b) an adsorbent (e.g., zeolite) or a catalyst-supporting adsorbent provided upstream of the purification catalyst (a) [reference is made to, for example, Japanese Patent Application Laid-Open Nos. 75327/1990, 173312/1990 and 135126/1990].
Further, an adsorbent comprising a metal carrier and zeolite coated thereon is disclosed in Japanese Patent Application Laid-Open No. 126937/1990.
The above proposals, however, each have a problem. The catalytic converter disclosed in Japanese Utility Model Application Laid-Open NO. 67609/1988 consists of a metal monolith catalyst as a preheater and a main monolith catalyst. With this catalytic converter, it is difficult to purify hydrocarbons in exhaust gas, at the start-up of an engine.
With the apparatus for purification of automobile exhaust gas arranged in the exhaust gas system of automobile (disclosed in Japanese Patent Application Laid-Open No. 75327/1990), comprising a purification catalyst and an adsorbent (e.g., zeolite) provided upstream of the purification catalyst, even if HC's are adsorbed by the adsorbent provided upstream of the purification catalyst, the HC's are desorbed from the adsorbent upon the warm-up of the engine. As a result, a considerable amount of untreated (unburnt) HC's pass through the purification catalyst which is not yet heated sufficiently.
Japanese Patent Application Laid-Open No. 173312/1990 discloses a technique utilizing a main exhaust gas passage containing a catalyst and a by-pass passage containing an adsorbent, wherein an exhaust gas is passed through the by-pass passage during the start-up of the engine, using a switching means and, when the temperature of the exhaust gas has reached the working temperature of the catalyst provided in the main passage, the exhaust gas is passed through the catalyst of the main passage using the switching means. With this technique, a complicated mechanism is required to enable the switching from the by-pass passage to the main passage when the catalyst in the main passage has been heated sufficiently. Moreover, a substantial amount of an exhaust gas passes through the catalyst of the main passage without being purified, before the catalyst is heated sufficiently.
In the apparatus for purification of automobile exhaust gas arranged in the exhaust gas system of an automobile (disclosed in Japanese Patent Application Laid-Open No. 135126/1990), comprising a purification catalyst and an adsorbent containing a catalyst supported thereon, provided upstream of the purification catalyst, the start-up of the purification catalyst is delayed because of the heat capacity of the adsorbent. Further, the amount of the catalyst added to the adsorbent is limited and sufficient purification is not possible.
Japanese Patent Application Laid-Open No. 125937/1990 discloses an adsorbent alone and mentions neither heater nor catalytic converter for exhaust gas including CO, HC's and NOX.
Furthermore, a zeolite used as an adsorbent in these references mentioned above is of Y type or mordenite. The zeolite is of poor heat resistance, and it adsorbs water contained in the exhaust gas, resulting in lowering of adsorptive power.